The present invention relates to a control apparatus for an image exposure apparatus which exposes an image on a printing plate, an image exposure apparatus, and a control apparatus which controls a multicolor printing press.
[Plate Making on Press]
In recent years, in order to improve the efficiency of plate making operation or to improve the registration accuracy, a plate making apparatus is attached to a printing press itself whereby plate making operation is directly performed on the printing press by the plate making apparatus. That is, instead of using a plate making apparatus separated from a printing press, a printing plate (raw plate) mounted on a plate cylinder is irradiated with a laser beam from the head of a plate making apparatus attached to a printing unit, thereby exposing an image. This operation is called plate making on press.
More specifically, the rotational speed of the printing press is increased to a designated value. When the rotational speed has stabilized, laser irradiation (exposure) from the head to the printing plate is started. After that, the head is moved in the axial direction of the plate cylinder while continuing exposure, thereby exposing an image on the entire plate mounted on the plate cylinder. The exposure time is determined by the plate size and the designated rotational speed at the time of exposure. Techniques for exposing an image on a printing plate by laser irradiation are disclosed in U.S. Pat. No. 5,379,698 (reference 1) and the like, and a detailed description thereof will be omitted.
FIG. 4 shows the attached state of plate making apparatuses to a four-color rotary printing press. Referring to FIG. 4, plate making apparatuses 102-1 to 102-4 are attached to printing units 101-1 to 101-4 of the respective colors. The plate making apparatuses 102-1 to 102-4 are normally at positions indicated by the alternate long and two dashed lines in FIG. 4. When exposure operation is to be performed, they are moved close to plate cylinders 103-1 to 103-4 in the printing units 101-1 to 101-4. Reference numerals 104-1 to 104-4 denote blanket cylinders on which blankets are mounted. Impression cylinders (not shown) are arranged under the blanket cylinders 104-1 to 104-4.
FIG. 5 shows main part of a plate making apparatus 102. The plate making apparatus 102 has an exposure unit 102b having a head 102a. The exposure unit 102b is fixed on a table 102c. The table 102c moves in the axial direction (indicated by a double-headed arrow A-B) of a plate cylinder 103 while being guided along rails 102f1 and 102f2 on a base 102f by a ball screw 102e rotated by a motor 102d. A printing plate (raw plate) 105 is mounted on the surface of the plate cylinder 103.
In plate making on press, the exposure range of an image onto the printing plate 105 is set before the start of actual image exposure by causing an operator to input the X-coordinate distance (X1,0) from the origin (0,0) at the left edge on the leading edge side of the printing plate 105 to the left edge of the image range and the Y-coordinate distance (0,Y1) to the leading edge of the image range, as shown in FIG. 6. That is, let W be the image size in the X-axis direction, and H be the image size in the Y-axis direction. The origin (0,0) is defined at the left edge on the leading edge side of the printing plate 105. The image range is defined by X-coordinates xe2x80x9cX1xe2x80x9d and xe2x80x9cX1+Wxe2x80x9d and Y-coordinates xe2x80x9cY1xe2x80x9d and xe2x80x9cY1+Hxe2x80x9d.
Assume that the number of pixels of the image is n in the X-axis direction and m in the Y-axis direction, as shown in FIG. 7. A distance xcex94X between the pixels in the X-axis direction is given by xcex94X=W/n, and a distance xcex94Y between the pixels in the Y-axis direction is given by xcex94Y=H/m. The plate making apparatus 102 defines xcex94X and xcex94Y as the exposure intervals in the X- and Y-axis directions and exposes image data that is input in advance within that image range.
More specifically, the head 102a of the plate making apparatus 102 is moved from the left to the right while rotating the plate cylinder 103 at a predetermined rotational speed. The head 102a is stopped at the position X1, and the pixels of one line in the Y direction are exposed at the interval xcex94Y. That is, pixels within the range from (X1,Y1) to (X1,Y1+H) are exposed. Next, the head 102a is moved to the right by xcex94X. At the next position, the pixels of the next line in the Y direction are exposed at the interval xcex94Y. This operation is repeated until the X-coordinate xe2x80x9cX1+Wxe2x80x9d.
The image data (image xe2x80x9c1xe2x80x9d/non-image xe2x80x9c0xe2x80x9d) of each pixel is not stored in correspondence with the data of its exposure position. Only data of image xe2x80x9c1xe2x80x9d/non-image xe2x80x9c0xe2x80x9d are sequentially stored. In actual exposure, the image data are sequentially read out, and the pixels are sequentially exposed from the position (X1,Y1) at the interval xcex94Y in the Y direction and at the interval xcex94X in the X direction. This is because the number of image data to be processed is enormous. If the image data are collated with position data and exposed one by one, a very long time and large storage capacity are impractically required.
In printing by a rotary printing press, a high pressure must be applied to printing paper between the blanket cylinder and the impression cylinder. For this reason, the printing paper stretches toward the trailing edge side. Hence, the image printed by the preceding printing unit expands into a wide trapezoidal shape toward the trailing edge side, resulting in misregistration between colors. This tendency is especially conspicuous in offset printing because printing is executed with water supplied.
FIG. 8 shows an image state on printing paper after printing of the second color. A printing paper sheet 106 stretches due to printing by the second-color printing unit, and a first-color image 107 expands into a trapezoidal shape. For this reason, shifts are generated between the first-color image 107 and a second-color image 108. That is, shifts w1 and w2 in the horizontal direction (a direction perpendicular to the sheet convey direction) of the printing paper sheet 106, a shift h in the vertical direction (sheet convey direction), and shifts (distortion amounts) s1 and s2 due to distortions are generated. Similarly, the first- and second-color images further expand into trapezoidal shapes due to printing by the third-color printing unit. The first-, second-, and third-color images further expand into trapezoidal shapes due to printing by the fourth-color printing unit. In this way, shifts are generated between the color images, resulting in a defective printing product.
To solve this problem, the present applicant proposed in Japanese Patent Laid-Open No. 2000-309084 (U.S. Pat. No. 6,283,467; reference 2) a sheet-like object convey apparatus which stretches the trailing edge side of a printing paper sheet in the horizontal direction (right-to-left direction) upon transferring the printing paper sheet to a printing section whereby the shape of the printing paper sheet is deformed in advance into a trapezoidal shape whose width increases toward the trailing edge side to eliminate or reduce the stretch of the printing paper sheet during printing, thereby eliminating or reducing the shift of the image due to distortion by the stretch of the printing paper sheet during printing. The correction operation of the sheet-like object convey apparatus disclosed in reference 2 will be described with reference to FIGS. 10 and 11.
Referring to FIG. 10, when a swing 1 pivots from a point b to a point a, i.e., the gripping position of a feeding cylinder 4 along with rotation of a feeding cylinder shaft 4a, the edge portion of a paper sheet 6 is gripped by a plurality of gripper units (not shown) each formed from a gripper and gripper pad. Simultaneously, the central portion of a support shaft (not shown) that supports the gripper units is pressed and deflected by xcex1, as indicated by the alternate long and two dashed line in FIG. 11. When the support shaft deflects, the gripper units at the central portion retreat from those on both sides by xcex1. In this state, when the feeding cylinder shaft 4a rotates to move the swing 1 from the point a to the point b, press against the support shaft is canceled. All the gripper units are aligned on one line, as indicated by the solid line in FIG. 11.
When the gripper units at the central portion move, the directions of gripper units are changed toward the left and right end sides of the paper sheet 6 from the central portion relatively to those in gripping the paper sheet. The paper sheet 6 is stretched to become wide toward the trailing edge side. With this operation, the paper sheet 6 is deformed in advance into a trapezoidal shape whose width increases toward the trailing edge side before printing. Since the stretch of the printing paper sheet during printing is eliminated or reduced, the shift of the image due to distortion by the stretch of the printing paper sheet during printing is eliminated or reduced. Hence, fan-out registration is corrected. Reference numeral 5 denotes a lower swing; 6a, a feedboard; and 7, an impression cylinder.
According to the sheet-like object convey apparatus described in reference 2, of the shifts of the image, the shifts s1 and s2 due to distortions are corrected, as shown in FIG. 9. However, since the shifts w1 and w2 in the horizontal direction and the shift h in the vertical direction cannot be corrected, defective printing products cannot be completely avoided.
It is an object of the present invention to provide a control apparatus for an image exposure apparatus, which eliminates misregistration between colors due to stretch of a printing paper sheet and prevents any defective printing product.
In order to achieve the above object, according to the present invention, there is provided an image exposure control apparatus comprising memory means for storing a correction amount for each color in accordance with a stretch amount of a printing paper sheet in multicolor printing operation, and adjustment means for adjusting an exposure position of a pixel of an image to be exposed for each color, on the basis of the correction amount read out from the memory means, in exposing the image on a printing plate.